Module 12 Routing Protocols

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Module 12Routing Protocols
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• Textbook sections
• BF Chapter 13 Routing Protocols (RIP,OSPF,and
  BGP)
• Topics
• RIP
• OSPF
• Overview
• Link-state Database
• Graphical Representation
• Shortest-path Tree and Routing Table Calculation
• OSPF Packets for link-state Database Updates
• BGP

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1. Routing Information Protocol (RIP)

• Use the Bellman-Ford algorithm to calculate
  routing tables
• RIP runs on top of UDP via well-known UDP port
  number 520.
• Initializing the routing table
• Updating the routing table
• RIP message format
• Remedies for instabilities

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Table A distance vector routing table
1. Routing Information Protocol (RIP)
Note Other information can include time since
last updated
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1. Routing Information Protocol (RIP)

• Initializing the routing table
• When a router is added to a network, it
  initializes a routing table for itself using its
  configuration file
• Destination field contains only the directly
  attached networks
• Hop count initialized to 1.
• The next hop field which identifies the next
  router, is empty.

BF Figure Initial routing tables in a small
autonomous system
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BF Figure Final routing tables for the previous
figure
Note Hop count is the number of networks a
packet enters to reach its final destination.
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1. Routing Information Protocol (RIP)

• Updating the routing table
• The routing table is updated upon receipt of a
  RIP response message

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1. Routing Information Protocol (RIP)

• Updating the routing table
• The routing table is updated upon receipt of a
  RIP response message

RIP Updating Algorithm
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BF Figure Example of updating a routing table
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1. RIP - RIP Message Format

• Command field
• Request
• When the value in command field is 1
• A request can ask about specific entries or all
  entries (Figure 13.7)
• Normally, requests are sent as broadcasts, from a
  UDP source port of 520.
• Response (Figure 13.8 Response message)
• Solicited response
• Response to a specific request
• It contains information about the destination
  specified in the corresponding request
• Unsolicited response
• Regular updates
• Triggered updates triggered by a metric change

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1. RIP - RIP Message Format

• Unsolicited response
• Regular updates
• Sent periodically, every 30 seconds trigged by a
  30 second timer
• The 30-second timer is offset by addition of a
  small random time each time its is set. (to
  prevent any possible synchronization and
  therefore overload on an internet if route
  updates simultaneously)
• Message contains the whole routing table
• Triggered updates triggered by a metric change
• Metric field
• This 32-bit field defines the hop count from the
  advertising router to the destination network
• Must contain a value between 1 and 15 inclusive,
  specifying the current metric for the
  destination, or the value of 16, which indicates
  that the destination is not reachable
• The maximum number of hops is limited to 15
  because RIP is intended to use in a local area
  environment. The metric value of 16 is reserved
  to represent infinity. This small number helps
  to combat the count-infinity problem.

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1. RIP - RIP Message Format
BF Figure RIP message format
Family field Define the the protocol family
under which the network address should be
interpreted. For TCP/IP the value is 2.
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1. RIP - RIP Message Format
BF Figure Request messages
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1. RIP - RIP Message Format
BF Figure Response message
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1. Routing Information Protocol (RIP)

• Remedies for instability
• Maximum metric value of 16
• Triggered update
• Split horizons
• Poison Reverse

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2. OSPF - Overview

• Overview
• In 1991, the industry moved to establish an open
  standard replacement for RIP. The result was
  OSPF (Open Shortest Path First).
• As the name implies, OSPF is an open standard
  used to seek out shortest path routes just like
  RIP
• OSPF operate under the link-state concept, in
  which each router keeps a link-state database, a
  topological database, of all links in a network
  and information on any delay it might be
  experiencing. Link-state protocols build routing
  tables based on a topology database.
• This database is built from link state packets
  that are passed between all the routers to
  describe the state of a network. OSPF saves
  control message overhead by issuing routing
  updates only on an event-driven basis.

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2. OSPF - Overview

• Highlights
• OSPF is a link-state routing protocol
• OSPF is designed to be run internal to a single
  Autonomous System. Distributes routing
  information between routers belongs to a single
  Autonomous System (AS).
• Each OSPF router maintains an identical database
  describing the Autonomous systems topology
• From this database, a routing table is calculated
  by constructing a shortest path tree
• Important features
• OSPF recalculates routes quickly in the face of
  topological changes, utilizing a minimum of
  routing protocol traffic (link-state updates)
• An area routing capability is provided, enabling
  an additional level of routing protection and a
  reduction in routing protocol traffic.
• Provides the authentication of routing updates
• Use IP multicast when sending/receiving the
  updates

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Highlights of OSPF Protocol
Graphical representation
Link-state Database
OSPF packets
Link-state algorithm
routing table
Shortest-path tree
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2. OSPF Link-state Database

• Link-state database
• A tabular representation of the topology of the
  internet inside an area. It shows the
  relationship between each router and its
  neighbors including the metrics.
• Highlights of Link-state database
• Each router maintain a database describing the
  Autonomous Systems topology
• Each participating router has an identical
  database
• Each individual piece of this database is a
  participants local state (e.g. the routers
  usable interfaces and reachable neighbors)
• The router distributes its local state throughout
  the AS by flooding
• All routers run the exact same algorithm in
  parallel.
• From the link-state database, each router
  constructs a tree of shortest path with itself as
  a root.

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Example of link-state database (a tabular
representation)
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2. OSPF Link-state Database

• Graphical representation
• Hierarchical routing
• One particular feature that makes OSPF more
  complex than other routing protocols also makes
  it more powerful
• To achieve a hierarchy, OSPF allows an autonomous
  system to be partitioned for routing purpose.
• A manager can divide routers and networks in an
  autonomous system into subsets that OSPF calls
  areas. Each router is configured to know the
  area boundary (i.e., exactly which other routers
  are in its area).
• When OSPF runs, routers within a given area
  exchange link-state message periodically.
• To exchange information with an area, OSPF allows
  communication between areas. One router in each
  area is configured to communicate with a router
  in one or more other area(s). The two routers
  summarize routing information they have learned
  from other routers within their respective area,
  and then exchange the summary.

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2. OSPF Link-state Database

• Thus, instead of broadcasting to all router in
  the autonomous system, OSPF limits link-state
  broadcasts to routers within an area. As a
  result of the hierarchy, OSPF can scale to handle
  much larger internets than other routing
  protocols.

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2. OSPF Link-state Database

• Autonomous system (AS)
• The global Internet topology can be viewed as a
  collection of Ass
• An AS is loosely defined as a set of routers or
  networks that are technically administered by a
  single organization such as a corporate network,
  a campus network, or an ISP network
• Each AS can choose an interior routing protocol
  to handle routing inside the AS.
• Categories of AS
• AS number

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2. OSPF Link-state Database
BF Figure Autonomous systems
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2. OSPF Link-state Database
BF Figure Areas in an autonomous system
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2. OSPF Link-state Database
OSPF Router Classification
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2. OSPF - Graphical Representation

• Types of links
• Point-to-point link
• Transient link
• A network with several routers attached to it.
• The data can enter through any of the routers and
  leave through any router.
• While there is a metric from each node to the
  designated router, there is no metric from the
  designated router to any other node. (The reason
  is that the designated router represents the
  network. We can only assign a cost to a packet
  that is passing through the network. We cannot
  charge for this twice.
• Stub link
• A network connected to only one router
• The data package enter the network through this
  single router and leave the network through this
  same router.
• A special case of the transient network
• Link is only one-directional, from the router to
  the network
• Virtual Link
• When the link between two routers is broken, the
  administration may create a virtual link between
  them using a longer path that probably goes
  through several routers.

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2. OSPF - Graphical Representation
BF Figure point-to-point link
BF Figure 13-21 Stub Link
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BF Figure Transient link
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2. OSPF - Graphical Representation

• Network types
• Point-to-point networks
• A network that joins a single pair of routers
• Example a 56kb serial line
• Broadcast networks
• Network supporting many (more than two) attached
  routers, together with the capability to address
  a single physical message to all of the attached
  routers (broadcast)
• Example An Ethernet
• Non-broadcast networks
• Network Supporting many (more than two) routers,
  but having no broadcast capability.
• OSPF runs in one of two modes over non-broadcast
  networks
• Non-broadcast multi-access(NBMA) mode which
  simulates the operation of OSPF on a broadcast
  network.
• Point-to-multipoint mode which treats the
  non-broadcast network as a collection of
  point-to-point links.

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2. OSPF - Graphical Representation

• OSPF Network topology
• OSPF requires a hierarchical network topology.
  The network can be broken up into areas.
• OSPF router classification
• Autonomous system boundary router (ASBR)
• Exchanges routing information with routers
  belongs to other autonomous systems
• Area border router (ABR)
• attaches to multiple areas and runs multiple
  copies of the routing algorithm, on for each
  attached area.
• Internal router
• has all directly connected networks belongs to
  the same area. It runs a single copy of the
  routing algorithm
• Backbone router
• has an interface to the backbone

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2. OSPF - Graphical Representation
BF Figure Example of an internet
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2. OSPF - Graphical Representation
BF Figure Graphical representation of an internet
Note metrics assigned to the designated node.
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2. OSPF - Shortest-path Tree and Routing Table
Calculation

• Dijkstra algorithm
• To calculate its shortest-path tree and routing
  table, each router applies the Dijkstra algorithm
  to its link state database

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2. OSPF OSPF Packets for link-state Database
Updates

• Operation
• OSPFs Hello protocol uses Hello packets to
  discover and maintain neighbor relationships
• The Database Description and Link State Request
  packets are used in the forming of adjacencies.
• OSPFs reliable update mechanism is implemented
  by the Link State Update and Link State
  Acknowledgement packets.
• OSPF packet types

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2. OSPF OSPF Packets for link-state Database
Updates

• OSPF common header
• All OSPF share the same common header
• Router ID Source router IP address (No
  destination IP address is included in the common
  header)
• This field defines the area within which the
  routing take place
• Checksum It does not include authentication type
  and authentication data field

0 8
16
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Version Type
Packet Length
Router ID
Area ID
Checksum
Authentication Type
Authentication
Authentication
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2.0 OSPF OSPF Packets for link-state Database
Updates
BF Figure Types of OSPF packets
Note Five different types of Link state
advertisements
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2.0 OSPF OSPF Packets for link-state Database
Updates

• Hello protocol
• The Hello protocol establishes and maintains
  neighbor relationships. Neighbor routers form
  adjacencies
• The Hello protocol is used to elect a designated
  router (DR) on multiaccess networks. Refer to RFC
  2328 Section 9.4 Electing the Designated router
  for details.
• Details
• Each router broadcasts a Hello packet
  periodically onto its network.
• When a router receives a Hello packet, it replies
  with a Hello packet containing the router ID of
  each neighbor it has seen.
• When a router receives a Hello packet containing
  its router ID in one of the neighbors fields, the
  router is assured that communication to the
  sender is bi-directional
• Designated routers are elected in each
  multiaccess network after neighbor discovery.
  The election is based on the priority and ID
  fields.

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2.0 OSPF OSPF Packets for link-state Database
Updates
LG Figure 8.35 OSPF Hello packet format (Not
including OSPF common header)
Note Hello interval The normal period, in
seconds, between hello messages. Dead interval
Gives a time in seconds after which a
non-responding neighbor is considered
dead. Neighbor n Gives the IP addresses of all
neighbors from which the sender has recently
received hello messages Priority is used in
selecting designated routers. Designated router
and Backup designated Router Contains IP
addresses that give the senders view of the
designated router and backup designated router
for the network over which the message is sent
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2.0 OSPF OSPF Packets for link-state Database
Updates

• Database description packet
• Packet contains link state header only
• The database description packet does not contain
  completely database information it only gives an
  outline, the title of each line in the database
• Client/server relationship
• When two routers want to exchange database
  description packet, one of them takes the role of
  master and the other the role of slave.
• Contents of the database can be divided into
  several messages.
• Example newly connected router
• After a router is connected to the system, it
  sends hello packets to greet its neighbors
• If this is the first time that the neighbors hear
  from the router, they send a database description
  packet
• The newly connected router examines the outline
  and finds out which lines of information it does
  not have.
• It then sends one or more link state request
  packets to get full information about that
  particular link.

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2. OSPF OSPF Packets for link-state Database
Updates
BF Figure Database Description Message
Note I flag is set to 1 if the message is the
first message M flag is set to 1 if this message
is not the last message. M/S flag indicates
whether the message is sent by a master (M/S 1)
or by a slave (M/S 0). Message sequence number
is used to match a request with the response Link
state header See BF Figure 13-37 LSA header
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2. OSPF OSPF Packets for link-state Database
Updates

• Link state request packet
• After exchanging database description messages
  with a neighbor, a router may discover that part
  of its link-state database are out of date.
• To request the neighbor supply updated
  information, the router sends a link state
  request message.

BF Figure 13-35 Link state request packet
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2. OSPF OSPF Packets for link-state Database
Updates

• Link state update packet
• Link state update packet is the heart of the OSPF
  operation. It is used by a router to advertise
  the state of its links
• Link state advertisements (LSA)
• Five different types
• Router link LSA
• Network link LSA
• Summary link to network LSA
• Summary link to AS boundary router LSA
• External link LSA
• Have common LSA headers with different bodies
• Reliable flooding

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2. OSPF - Reliable flooding

• OSPF uses reliable flooding to ensure that LSA
  are updated correctly.
• Suppose that a routers local state has changed,
  so the router wishes to update its LSA. The
  router then issues a link-state update packet
  that invokes the reliable flooding procedure.
• Upon receiving such a packet, a neighbor router
  examines the LSAs in the update. The neighbor
  installs each LSA in the update that is more
  recent than those in its database and then sends
  an LSA acknowledgement packet back to the router.
  These packets consists of a list of LSA headers
• The router also prepares a new link-state update
  packet that contains the LSA and floods the
  packet on all interfaces other than the one in
  which the LSA arrived.
• All routers eventually receive the update LSA.
• Each router retransmits an LSA that has been sent
  to a neighbors periodically until receiving
  corresponding acknowledgement from the neighbor.

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BF Figure Link state update packet
OSPF common header
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BF Figure LSA header
Link state age Number of seconds elapsed since
this message was first generated. E indicates
the area is a stub area T indicates that the
router can handle multiple types of service. Link
state type Defines LSA type. Link state ID An
IP address. The value of this field depends on
the type of link. Advertising router The
IP address of the router advertising this
message. Link state sequence number A number
assigned to each link state update message. Link
state checksum Fletchers checksum based on the
whole packet except for the age field. Length
Defines the length of the whole packet in bytes.
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2.0 OSPF OSPF packets for link-state updates
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Figure 13-24
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BF Figure Router link
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BF Figure Network link
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BF Figure Summary link to network
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BF Figure Summary link to AS boundary router
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BF Figure External link
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BF Figure Router link LSA
Link ID Defined in BF Table 13.3 Link data
Defined in BF Table 13.3 Link type Defined in BF
Table 13.3 of TOS Defines the number of types
of service (TOS) announced for each link TOS
Defines the type of service Metric Define the
metric for the corresponding TOS E flag is set
to 1 if the advertising router is an autonomous
boundary router (ABR) (E stands for external) B
flag is set to 1 if the advertising router is an
area border router
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Table Link types, link identification, and link
data for router link LSA
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BF Figure Link state acknowledgement packet
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2. OSPF

• Comparing OSPF with RIP (OSPF is an enhancement
  over RIP)

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3. BGP

• The Border Gateway Protocol version4 is the de
  facto standard interAS (inter-autonomous system)
  routing protocol in todays Internet.
• Defined in
• RFC 1771 A Boarder Gateway Protocol 4 (BGP-4)
• RFC 1772 Application of the Border Gateway
  Protocol in the Internet
• RFC 1773 Experience with the BGP-4 protocol
• RFC 1774 BGP-4 Protocol Analysis
• BGP has been designed to allow many kinds of
  routing policies to be enforced in the interAS
  traffic.
• Typical policies involve political, security, or
  economic considerations

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3. BGP

• BGP can be characterized as a path vector
  protocol
• Propagate path information, such as the sequence
  of ASs on a route to a destination AS
• Does not specify how a specific route to a
  particular destination should be chosen among the
  routes that have been advertised.
• BGP provides the mechanism to distribute path
  information among the interconnected AS, but
  leaves the policy for making the actual route
  selections up the network administrator.
• Policies are manually configured into each BGP
  router. They are not part of the protocol
  itself.
• BGP peers
• BGP uses TCP as its transport protocol rather
  than defining its own
• After a neighbor BGP session is established, it
  remains open unless specifically closed or unless
  there is a link failure.
• When tow neighbor routers are exchanging BGP
  session and route information, they are said to
  be BGP peers.
• The route information exchanged between peers
  includes the network number/AS path pair and
  other route attributes